The importance of and need for high speed and high resolution recording instrumentation is well known. Furthermore, electronic analog-to-digital converters (ADC) are known to sample and perform quantization in the amplitude domain (i.e. quantization of amplitude information) which places a tremendous burden on amplitude precision and fidelity. However, measurement precision in the amplitude domain can be limited by the amplitude resolution of the quantizer. Furthermore, ADCs are known to exhibit several noise sources inherent to the quantization process, including thermal noise, sampling aperture jitter, and comparator ambiguity. Electronic sampling jitter in particular can cause the amplitude noise of an ADC to increase with input frequency, limiting their usefulness in high speed and high precision applications. These effects combine to limit the overall performance of the recorder, i.e. effective number of bits (ENOB), which is exacerbated at higher speeds. For example, a fast 20 GS/s commercially available oscilloscope can provide about ˜32 resolvable levels across a 10 GHz bandwidth, which may be insufficient for many applications, such as for example, wideband communication, remote sensing, and scientific research. Electron gun/tube-based oscilloscopes and streak cameras offer high dynamic range but have very limited record length and will not produce single-shot measurements at high repetition rates or for continuous data.
Various photonic methods for improving the performance of ADCs using amplitude quantization are known. For example, photonic time-stretch ADC operates by slowing down RF signals so they can be digitized with higher resolution, low bandwidth ADCs. While the time-stretch approach works well to achieve bandwidth reduction and reduce noise, the technique can introduce unwanted distortions that limit performance. For example, in the time domain, the sinusoidal (Mach-Zehnder) MZ transfer function requires linearization. In the frequency domain, chromatic dispersion induced fading requires compensation. To increase the record length, individual time segments require high fidelity stitching. And time warps in the stretched RF signal can be induced by wavelength dependent bias offsets of the MZ modulator, higher order dispersion terms in the optical fiber, and wavelength dependent group delay variation in the wavelength division multiplexers (WDMs). Each of these distortions can add complexity and limit the overall resolution of the system.
Another method known as optically-sampled ADCs performs sampling in the optical domain and amplitude quantization in the electronic domain. This technique overcomes the electronic jitter noise of ADCs due to superior jitter properties of mode-locked lasers and the use of optical sampling. However, accurate digitization is not assured because other imperfections, such as for example noise from photodetectors, RF amplifiers, laser amplitude fluctuations, and individual electronic ADCs as well as nonlinear distortions from the MZ modulator, photodetectors, and RF amplifiers, can potentially limit the system's performance.